Many applications require the placement of a number of closely spaced beads of viscous liquid such as hot melt thermoplastic material. For example, in securing the flaps of cartons or other paper products, or in attaching the backing sheet to the non-woven layer of a hygienic article, rows of closely spaced hot melt adhesive beads are intermittently applied to such substrates to provide the desired bond. In applications of this type, the problems which have confronted designers include the accurate placement of a large number of relatively thin beads in a small area and a capability of turning on and off the flow of such beads without "cut off drool", i.e., the formation of elongated strands or strings after flow of the material is terminated.
One approach to the problem of locating a number of thin beads in a small area is disclosed, for example, in U.S. Pat. Nos. 3,570,725 and 3,840,158. In these patents, a number of dispensing devices or guns each having a nozzle are oriented with respect to a moving substrate such that each dispenser discharges a separate bead onto a common target area. In the U.S. Pat. No. 3,570,725, each dispenser or gun is fed by a separate adhesive line, whereas the U.S. Pat. No. 3,840,158 discloses a number of small guns mounted to a common source or manifold from which the adhesive is supplied. The problem with systems of the type described in these patents is that a relatively large amount of hardware is required in order to supply adhesive to the dispensers and provide controls for turning on and off the dispensers to obtain intermittent application of the adhesive beads. Additionally, the spacing between adjacent beads is limited by the size of the dispensers, which, in many applications, produces a wider spacing between beads than is needed.
With respect to the problem of cut off drool resulting from intermittent operation of adhesive dispensers, systems have been proposed of the type disclosed, for example, in U.S. Pat. Nos. 4,687,137, owned by the assignee of this invention. U.S. Pat. No. 4,687,137 to Boger et al discloses an apparatus for dispensing beads of thermoplastic adhesive onto the non-woven layer of disposable diapers using a "coat hanger" die nozzle comprising a pair of die halves which together form channels for transmitting separate streams of adhesive to discharge outlets from which the adhesive is extruded as beads onto a substrate. While this apparatus can be operated intermittently with minimal cut off drool, coat hanger die nozzles are relatively expensive to produce and can clog or plug if the adhesive becomes contaminated with particles or the like. In the event of a clog, it is time-consuming to disassemble and clean such coat hanger dies.